The following abbreviations used in the specification and/or the drawings are defined as follows:
3GPP third generation partnership project
CSG closed subscriber group
DL downlink (network towards UE)
eNodeB base station of a LTE/LTE-A system
E-UTRAN evolved universal terrestrial radio access network
ID identifier
LTE long term evolution (of the E-UTRAN system)
MAC medium access control
MME mobility management entity
PLMN public land mobile network
S-GW serving gateway
SCH shared channel
RRC radio resource control
UE user equipment
UL uplink (UE towards network)
In the E-UTRAN system relevant to these teachings there are conventional network access nodes/cells termed eNodeBs which serve all users, and also subscriber group CSG cells such as home eNodeBs which are available for traffic (voice and/or data) to only those subscribers registered with the CSG and possibly also certain allowed guests. Any given CSG may include a group of cells (such as a corporate or university campus) or a single cell. CSGs may allow traffic access for non-subscribers for emergency calls but these teachings relate to routine, non-emergency access.
These different types of cells or access nodes may be termed more generally as public access nodes/public cells and private access nodes/private cells. Other wireless systems (GERAN, GSM, UTRAN, WCDMA) have either implemented or are considering implementing similar such private networks as more functionality is shifted from higher in the radio access network toward the base stations/access nodes.
In the LTE system the UE is to keep a list, termed a CSG whitelist or CSG allowed list, which recites all the identities of the CSG cells for which that particular UE has access rights. The wireless specifications at 3GPP TS 36.300 for LTE Release 9 stipulate that an E-UTRAN network may configure a UE to report that the UE has detected that it has entered or left the proximity of a CSG cell to which the UE is allowed to connect. In the art these are termed proximity indications, and their purpose is to help to facilitate UE handovers from macro cells to CSG cells. Such handovers tend to improve the efficient use of network resources since after the handover that UE's traffic is offloaded from the conventional cellular network macro cells to instead pass through the CSG cell(s).
More specifically, 3GPP TS 23.401 states that the UE is to keep this CSG whitelist, which can be either the “allowed CSG list” or the “operator CSG list” contained in the so-called UE context which is provided by the UE's home network to any other networks the UE is transiting. Each CSG list has the form of a list of CSG IDs and the associated PLMNs. Each CSG ID reliably identifies a CSG, which as above might include a single CSG cell or multiple CSG cells.
FIG. 1 illustrates a relevant wireless environment; a UE 20 is connected to a macro cell 22 and moving toward a CSG cell 24. While only one is shown, in practice there may be many CSG cells from different CSGs and the UE may be a subscriber to only one or a few of those CSGs. In order for the UE 20 to determine whether or not it has access rights to that CSG cell 24 it must acquire the CSG ID of that CSG in order to check it against the UE's own CSG whitelist.
The most accurate way to determine the CSG ID of a CSG cell 24 is for the UE 20 to read the system information of that cell 24. The UE 20 will read the CSG ID from the System Information Block Type1 message which is part of the system information broadcast by the CSG cell 24 on the DL-SCH as set forth at 3GPP TS 36.331. Herein lies a problem; reading system information of a neighbor (non-serving) cell means the UE cannot at the same time remain in contact with the macro (serving) cell 22 to which it is in a connected state (assuming as is typical that the UE is using the same radio receiver to tune between the CSG cell's broadcast channel and the macro cell's DL control channel over which resource allocations or pages are sent). If there were only one CSG cell 24 as in FIG. 1 the problem is minor, but where there are multiple CSG cells this can easily result in disrupting normal communications between the UE 20 and its connected macro cell 22. The inventors consider that it is not desirable to require UEs to read the system information of every neighbor cell whose broadcast channel they can hear, for this makes the UE less available to its serving cell for regular data transmissions.